Increase compatibility with single threaded Runtimes and LocalSets.

[Aurora2500]

The use of block_in_place can be problematic as it is not allowed to be ran inside a single threaded runtime or inside a LocalSet.

For example, these are important in ecosystems like Actix which has decided to have !Send futures and prefer to use single threaded runtimes. Making the use of this library impossible in these ecosystems.

[jaboatman]

To do so would require scope to be asynchronous, which is unsafe (see #3) - really, I made this library years ago for a specific purpose at work and I don't use it otherwise. async_scoped has support for what you want and is probably a better choice overall.